1. Field of the Invention
The invention relates to rotary drill bits for drilling in subsurface formations and of the kind comprising a bit body having a leading surface formed at least in part from solid infiltrated matrix material, at least a major part of the leading surface being impregnated with a plurality of abrasive particles of superhard material. Such bits are commonly referred to as impregnated or "impreg" bits.
2. Description of Related Art
As is well known, such a drill bit usually comprises a steel core around which the main part of the bit body, providing its leading face, is formed by a powder metallurgy process. In this process the steel core is located in an appropriately shaped mould which is then packed with particulate matrix-forming material, usually powdered tungsten carbide. A solid body of suitable copper or other alloy is placed above the packed particulate material and the whole assembly is placed in a furnace so that the alloy fuses and infiltrates downwardly through the carbide particles so as to form, upon cooling, a body of solid infiltrated matrix material in the shape of the mould. The abrasive particles with which the matrix material is impregnated commonly comprise small bodies of natural or synthetic diamond, the latter usually being in the form of single crystals although bodies of thermally stable polycrystalline diamond may also be employed. The abrasive particles are located within appropriate parts of the mould before it is packed with the matrix-forming particles.
Such impregnated drill bits are particularly suitable for drilling through very hard subsurface formations. However, when drilling a borehole, the situation often arises where a partly completed borehole is wholly or partly blocked and it is necessary to drill out the blockage before a new portion of the borehole can be drilled. Thus it may be necessary to drill out items such as plugs, floats, float collars, shoes, shoetracks or liner hanger equipment. For example, in order to inject cement into the spaces between the casing of a section of borehole and the surrounding formation it is common to pump the cement down the interior of the casing followed by a column of drilling fluid, so that the pressure of the drilling fluid forces the cement upwardly around the casing from below. A shoetrack is a device, formed mainly from aluminium, rubber and cement, which is used to separate the drilling fluid from the cement, and which remains at the bottom of the borehole section, blocking it, after the cementing operation has been completed. The shoetrack must therefore be drilled out before drilling of a further section of the borehole can be resumed.
However, the cutting structure of an impregnated drill bit is not suitable for the rapid drilling out of temporary obstructions in the borehole of the kind described above, being designed to perform a comparatively slow grinding away of very hard subsurface formations. Hitherto therefore, where it has been desired to use an impregnated bit to drill the borehole, it has been necessary to drill out the shoetrack, or other blocking structure in the borehole, with a different type of drill bit before continuing to drill the borehole itself with the impregnated bit. The tripping of a drill bit into and out of an existing borehole is costly and it would therefore be advantageous to employ a drill bit which is capable both of drilling out the shoetrack or other obstruction and then continuing to drill the borehole in the formation. However, conventional drill bits which may be capable of drilling out the obstruction, such as some types of drag-type drill bits or roller cone bits, may be much less effective than an impregnated bit for subsequently drilling the hard formation.
The present invention therefore sets out to provide an improved form of impregnated drill bit which may also be capable of drilling out shoetracks or similar devices which may temporarily obstruct a borehole.